The Navy currently has only one topside coating that is qualified for use under MIL-PRF-24635. This coating, which is a silicone alkyd, has been used by the Navy for over 50 years, even though the technology continues to provide poor exterior durability (color and gloss retention upon exposure to an oxidative environment with UV and visible light), poor chemical resistance, low hardness, slow cure times and limited cleanability. The formulation of low volatile organic component (“VOC”) versions has also recently become difficult, as the utilization of lower viscosity raw materials are increased in effort to maintain sprayable and/or rollable formulations. Unfortunately, this continued decrease in viscosity is typically accompanied by a reduction in resin molecular weight and cross-linkable functionality, thereby reducing the cross-link density and hardness of the resulting cured coating. A reduction in the molecular weight of resin components can also lead to sagging of the coating when applied. The VOC of silicone alkyd formulations has also been lowered by using exempt solvents, such as Oxsol 100, which do not count against the VOC rule. Although these solvents do not contribute to ozone depletion, it remains beneficial for paint applicators to use coatings with the lowest solvent content possible.
Silicone alkyd coatings can be formulated as single-component (also known as “1K”) systems because their cross-linking mechanism occurs as a result of atmospheric oxygen reacting with residual alkenes contained within the resins of the coating. The coating does not begin to cure until it is applied on a surface and the solvent evaporates, thereby giving it essentially a limitless pot-life, unlike two-component (2K) systems that begin to react once the components are mixed. Because it is a single-component system, no specialty mixing, equipment, or training is needed to apply the coating, as it can be easily spray-applied by individuals during depot level painting. Navy sailors are routinely found applying (via roller or brush) silicone alkyd coatings to a ship for the sake of touch-up and/or beauty coating, yet this mundane and non war-related task would not be required if the silicone alkyd coatings already on the ship did not fade, chalk, discolor or stain within only a few months after application.
Acrylic siloxanes are conventionally used as both additives and base resins in commercial coatings. These resins are easily manufactured via polymerization of alkoxysilane-functional acrylate monomers with ethyl, butyl, or hexyl acrylate and/or meth(acrylate) monomers. Depending on the molecular weight, acrylic siloxane resins may contain moderate-to-high VOC levels, yet because the alkoxysilane groups are the only reactive functionality, these resins can be used to generate single-component (1K) coatings. Nevertheless, while these moisture curable resins are found in commercial 1K coatings, they are not without their drawbacks. The biggest problem with acrylic siloxanes is that they are slow to cure when not exposed to high humidity environments.